1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor for use in printers, digital copying machines, facsimiles, etc., and to a method for forming an electrophotographic image.
2. Description of the Related Art
At present, an electrophotographic system typified by the Carlson system has been widely used for its high-speed recording ability, high image quality, and non-impact property. In the electrophotographic system at present, (a) uniformly charging a photoreceptor, (b) writing image information by light (forming a latent image), (c) forming a toner image with a developer, (d) transferring the toner image onto a plain paper, and (e) fixing the toner image are fundamental processes. Recently, in particular, with the employment of image information as a digital information, a system in which laser light or LED array light is employed as a light source in place of white light in the prior art systems has been widely used and thereby an output image of higher image quality has been demanded.
In response to the demand, in an optical recording head and an optical recording system, a development has been proceeded aiming at a high-speed property and the increase of resolution, and for example, variable spot laser recording system (O plus E, 1996, May), multilaser beam recording system, an LED print head for 1200 dpi resolution, and further super-precise and super-high-speed polygon mirror (Japan Hard Copy '96, theses collection), etc., have been developed.
Because in the system in which such a semiconductor laser or an LED array is used as a recording light source, an image is represented by the assembly or array of fine dots called pixels, a technique of forming fine spots by increasing the resolving power of the optical system becomes inevitable and consequently in the optical system side, a recording density of 1200 dpi or more has become possible.
Also, what is important for obtaining high image quality is not only the optical system, but also, together with techniques of reducing the particle sizes of toner particles and minimizing the occurrence of scattering of toner at the development or transferring stages, an image-processing technique which processes image data to match the image-reproducing characteristics of the electrophotography.
As the technique of reducing the particle sizes of toner particles, for example, Japanese Examined Patent Publication JP-B2 2696400 discloses an image forming method in which digital exposure is carried out at a recording density of 600 dpi or more and a toner having a particle size of 8 .mu.m or less is used. However, it has been known that in the case of forming a high quality digital image having a recording density of 1200 dpi or more, it is difficult to faithfully reproduce, only by limiting the weight average particle size of a toner, an electrostatic latent image formed on a photoreceptor, and it is necessary to design the photoreceptor so as to incur no deterioration of the recording density.
In general, visually observed image quality is determined by the synergistic effect of the resolution and the gradient, and prints of a work of art etc. have a resolution of at most 200 dpi but an image of high quality is obtained because a 256-step tone can be expressed. It is said that human eyes have the faculty to detect a resolution of 300 dpi and a density of a 64-step tone, and so it is clear that in terms of gradation, in area gradation, high resolution is advantageous and in density gradation, the number of steps in tone may be low, and an image forming method considering this balance becomes important. Taking into consideration the stability of images with the passage of time, stability of images in a variety of environments, etc., and when the instability of halftone density in the electrophotography is considered, for realizing a high quality stable image, in dealing with gradation it is most advantageous to increase the resolution of the area gradation.
Also, the report "Increase of Image Quality of Electrophotography--Digital Recording Technique" in Electrophotography (The Society Journal, Society of Electrophotography of Japan), Vol. 26, No. 1, (1987), a technical explanation of method of increasing the image quality of electrophotography. In the report, it is stated that when the pulse width modulation method is used as a laser multivalue output method, the peak value of the light energy distribution is lowered, and because the distribution is linked with the intensity modulation characteristics, the electrostatic latent image potential distribution shows an intermediate value between a dark charging potential and a bright surface potential. However, it is clear that as recording comes to have higher resolution and as recording of more values is carried out, a photoreceptor having high sensitivity and high resolution becomes necessary.
However, investigations into the relation between a photoreceptor itself and the resolution, or the relation between a photoreceptor used in a recording system and the resolution are rare and the resolution of a photoreceptor itself has not hitherto been treated as a problem. This is because in a recording density of from 400 dpi to 600 dpi, a photoreceptor of the film thickness which has hitherto has been in practical use has sufficient resolution, and the deterioration of the resolution based on carrier diffusion due to the film thickness does not become a problem. Rather, for increasing the sensitivity as well as for prolonging the life of image, a photoreceptor having a thicker film thickness has been investigated thereof. For example, in Japanese Unexamined Patent Publication JP-A 7-244388 (1995) and 7-261415 (1995), techniques of increase the film thickness of photoreceptor to 27 .mu.m or thicker are reported.
On the other hand, for the performance required of a photoreceptor employed for digital recording, there are the following new requirements. When digital image formation is carried out by directly utilizing information from a computer, information such as a letter is converted into a light signal and is recorded on a photoreceptor. On the other hand, when digital image processing is carried out by inputting information from a manuscript, after reading the manuscript information as light information, the light information is converted into a digital electric signal, which is converted again into a light signal and is recorded on a photoreceptor. In each case, the information is recorded on the photoreceptor as the light information, and as chief recording means, a laser light or an LED light is used. The recording means which is used well at present is a near infrared light source having a wavelength of 780 nm or an infrared light source having a wavelength of 650 nm. The photoreceptor for digital recording is required to have high sensitivity to these light sources, and crystal-type phthalocyanine-base compounds have been widely investigated and have been practically used. For example, in Japanese Patent No. JP-C 2073696 (JP-B2 5-55860), a photoreceptor using a titanyl phthalocyanine is described, in Japanese Unexamined Patent Publication JP-A 59-155851 (1984), a photoreceptor using .beta.-type indium phthalocyanine is described, and in Japanese Unexamined Patent Publication JP-A 61-28557 (1986), a photoreceptor using vanadium phthalocyanine is described.
Also, it is reported that in the case of a crystal-type titanyl phthalocyanine, there exist various crystal systems and according to a difference in the crystal systems, there are large differences in a charging property, dark decay, sensitivity, etc. Particularly, in these phthalocyanines, the investigation of oxotitanyl phthalocyanine, which shows high sensitivity, has been vigorously carried out. Oxotitanyl phthalocyanine alone has been classified into many crystal types from a difference in diffraction angles of X-ray diffraction spectra as described in Electrophotography (The Society Journal, Society of Electrophotography of Japan), Vol. 32, No. 3, p. 289. Specifically, characteristic crystals are shown as follows. An .alpha.-type crystal is described in Japanese patent No. JP-C 2007449 (JP-B2 6-29975), an A-type crystal in Japanese Patent No. JP-C 1917796 (JP-B2 5-31137), C-type crystals in Japanese Patent No. JP-C 1876697 (JP-B2 6-1386) and 1997269 (JP-B2 7-30267), Y-type crystals in Japanese Patent No. JP-C 1950255 (JP-B2 6-39575) and 2128593 (JP-B2 7-91486), an M-.alpha.-type crystal in Japanese Examined Patent Publication JP-B2 7-15067 (1995), I-type crystals in Japanese Examined Patent Publication JP-B2 2502404, and an M-type crystal in Japanese Patent No. JP-C 1978469 (JP-B2 7-5851). Furthermore, in Japanese Examined Patent Publication B-2 2700859 and Japanese Unexamined Patent Publication JP-A 8-209023 (1996), crystals fundamentally classified in the Y-type crystal are described.
In crystal-type oxotitanyl phthalocyanine, there are various crystal systems, and accordingly it is necessary to specify the configuration of the crystal lattice of the oxotitanyl phthalocyanine is required. It is reported that in the crystal-type oxotitanyl phthalocyanine, according to the differences in such crystal systems, there are large differences in the charging property, the dark decay, the sensitivity, etc.
As described above, in order to increase image quality, investigation into increasing the resolution of the photoreceptor itself is required and also the investigation of a highly sensitive photoreceptor faithfully reproducing high-density recording of from 1500 dpi to 2400 dpi is required. In recording density of 600 dpi or lower at present, the film thickness of the photoreceptor practically used is from 20 to 35 .mu.m. The film thickness has been established in consideration of the sensitivity, printing durability (the life of the photoreceptor), etc., required at the photoreceptor. A latent image formed on the photoreceptor does not give a problem with regard to the reproducibility of the recording density. However, in the case of the latent image having a high density of 1500 dpi or higher, because in the photoreceptor having a film thickness of 20 .mu.m or thicker, carrier diffusion occurs due to the carrier traveling distance of the photoreceptor to cause the deterioration of the resolution, a problem that the faithful reproduction of the image becomes difficult has been resolved by the present inventors' investigations.
Also, for preventing the deterioration of the resolution of the latent image formed on a photoreceptor which is required to have high resolution, it becomes necessary to increase the surface charge density and to thin the film thickness of the photoreceptor to the extent that the deterioration through the diffusion of the carrier does not cause a problem. However, when the film thickness of a photoreceptor is thin, electric field intensity applied to a photosensitive layer is increased and thus there occur new problems such as the need for improving the pressure resistance of the photosensitive layer, and the loss of effective sensitivity accompanied by the increase of electric capacity. The problem of pressure resistance causes the formation of minute defects in the image through reversal development, and the problem of loss of effective sensitivity causes lowering of potential contrast and also causes a vicious circle of further increasing both a surface potential and the power of the recording light source, needed to ensure sufficient image density.